Seamless laser ablated roll tooling

ABSTRACT

A system for generating a laser machined tool from a substantially cylindrical work piece. The system includes a laser producing a laser beam, a mask positioned within the laser beam for shaping the laser beam into an image, and an optical system for imaging the laser beam image onto the outer surface of the work piece. The system coordinates rotational and translation movements of the work piece with activation of the laser in order to use the laser image for ablating the outer surface of the work piece, creating microstructures within the surface of the work piece to form the cylindrical tool.

BACKGROUND

Platforms have been developed for laser ablation machining for creatingcomplex micron scale structured surface tooling on a flat polymer sheet.These platforms use excimer lasers to ablate polymer sheets that areheld to a vacuum chuck. An optical train controls the laser beam andimages a mask onto the surface of the polymer, ablating a pattern thatis controlled by the design of the mask. These systems have proven thecapability to produce a wide variety of structures with mechanical andoptical properties. The structures created on these platforms can beused to create flat replicates for prototypes. Roll tools can be createdfrom the flat tools by welding a nickel copy of the polymer into acylindrical sleeve. Such a sleeve will have a seam in it, which can beundesirable when making films from the roll tools.

A need exists for additional ways to make a roll tool using laserablation, in particular, a cylindrical tool without a seam.

SUMMARY

A first system, consistent with the present invention, can generate alaser machined tool from a substantially cylindrical work piece. Thesystem includes a laser producing a laser beam and an optical system forprocessing the laser beam image and for imaging the processed laser beamimage onto the outer surface of the work piece. The processing of thelaser beam image is related to a curvature of the outer surface of thework piece and provides a way to accurately image onto a curved surface.The system rotates the work piece and uses the laser beam image forablating the outer surface of the work piece in order to createmicrostructures within the surface to form a substantially cylindricaltool.

A second system, consistent with the present invention, can generate alaser machined tool from a substantially cylindrical work piece. Thesystem includes a laser producing a laser beam and an optical system forimaging the laser beam image onto the outer surface of the work piece.The optical system provides for a deviation of less than 20 microns ofthe laser image in a direction perpendicular to the outer surface of thework piece. The system coordinates rotational and translation movementsof the work piece with activation of the laser to use the laser imagefor ablating the outer surface of the work piece in order to createmicrostructures within the surface to form a substantially cylindricaltool.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated in and constitute a part ofthis specification and, together with the description, explain theadvantages and principles of the invention. In the drawings,

FIG. 1 is a diagram of a system for laser machining of a cylindricalwork piece;

FIG. 2 is a diagram illustrating use of optics and a mask to form animage on the work piece for machining of it; and

FIG. 3 is a diagram illustrating an image formed on the work piece.

DETAILED DESCRIPTION

Laser Machining System

The laser machining system can be used to create a polymer roll tool vialaser ablation. As described below, a roll based laser ablation system;for example, an excimer laser, an optical system, and a work piececomprising a roll coated with a machinable material such as a polymer.The optical system can include, as described below, an optical train, aprojection mask supporting system, and imaging optics. Other possibleoptical systems can use mirrors or holographic techniques to perform theimaging and ablation described below.

FIG. 1 is a diagram of an exemplary laser machining system 10 formachining a roll tool, referred to as a work piece. The machining caninclude, for example, making microstructures in the work piece.Microstructures can include any type, shape, and dimension of structureson, indenting into, or protruding from the surface of an article.

The terms “microstructure” or “microstructures” refers to structureshaving at least one dimension (e.g., height, length, width, or diameter)of less than 2 millimeters (mm) and more preferably less than 1 mm.Microstructures created using the system described in the presentspecification can have a 1000 micron pitch, 100 micron pitch, 1 micronpitch, or even a sub-optical wavelength pitch around 200 nanometers(nm). These dimensions are provided for illustrative purposes only, andmicrostructures made using the system described in the presentspecification can have any dimension within the range capable of beingtooled using the system.

System 10 is controlled by a computer 12. Computer 12 has, for example,the following components: a memory 14 storing one or more applications16; a secondary storage 18 providing for non-volatile storage ofinformation; an input device 20 for receiving information or commands; aprocessor 22 for executing applications stored in memory 16 or secondarystorage 18, or received from another source; a display device 24 foroutputting a visual display of information; and an output device 26 foroutputting information in other forms such as speakers for audioinformation or a printer for a hardcopy of information.

The machining of a work piece 32 is performed by an excimer laser 28along with an optical system. The optical system in this exampleincludes optics and projection mask 30, which selectively blocksportions of a laser beam 31 forming an image for machining a material onwork piece 32. Under control of computer 12, laser 28 can provide pulsesof laser beam 31. For machining of the outer surface of work piece 32,the laser, optics, and projection mask are generally held stationarywhile the work piece 32 rotates and translates in a directionsubstantially perpendicular to laser beam 31. In particular, a driveunit 36, under control of computer 12, rotates work piece 32 in adirection shown by arrow 33 or a reverse direction. For the translationof work piece 32, it is supported by mounts 37 and 39, which can movework piece laterally along a track 40 in a direction shown by arrow 34using a drive unit 38 under control of computer 12. Mounts 37 and 39,via drive unit 38, can also be configured to move work piece in adirection substantially parallel to laser beam 31 as shown by arrow 35.The movement of work piece 32 in the direction shown by arrow 35 can beused to assist in precisely focusing the image formed by laser beam 31onto the outer surface of work piece 32. Alternatively, the laser 28 andwork piece 32 can be held stationary, except for rotation of work piece32 as illustrated by arrow 33, while the optics and mask 30 translatealong the work piece. In this alternative embodiment, system 10 can alsobe configured for path length compensation of the image generated bylaser 28 and optics and mask 30 when they translate along work piece 32.

FIG. 2 is a diagram illustrating use of optics and a projection mask toform an image on the work piece for machining of it. Optics 42 providelaser beam 31 to an imaging mask 44. Based upon a configuration ofimaging mask 44, an image of laser beam 31 is provided to imaging optics46 in order to project the image 48 onto the machinable material 41 ofwork piece 32. System 10 uses imaging, rather than spot writing, formachining of work piece 32. Several clear distinctions exist betweenspot writing and imaging. In spot writing, a system works at the focalpoint of a lens. In imaging, a system works at the image of a projectionmask. An overlap technique is referred to as shaped spot writing, whichis an imaging technique using a projection mask and imaging lens to makea simple shape with the laser beam, such as a triangle or crescent, andmoving that shape to make a desired shape for machining. In spotwriting, the pixel is the beam, usually round and as small as possible.In shaped spot writing, the pixel is a shaped spot, although much largerthan the smallest spot possible with the laser beam. In imaging, thepixel is typically the smallest spot possible, but these pixels are allcombined into one desired imaging structure for ablation.

Work piece 32 is typically implemented with a metal roll coated with alaser machinable polymeric material. The metal roll can be implemented,for example, with hard copper or with steel coated with nickel orchrome. Work piece 32 can be alternatively implemented with aluminum,nickel, steel, or plastics (e.g., acrylics) coated with the machinablepolymeric material. Examples of such polymeric materials for machiningare described in U.S. patent application Ser. Nos. 11/278,278 and11/278,290, both of which were filed Mar. 31, 2006 and are incorporatedherein by reference as if fully set forth. The particular material to beused may depend upon, for example, a particular desired application suchas various films made using the machined work piece. The machinablepolymeric material can be implemented with, for example, polyimide andurethane acrylate. A diamond-like-glass (DLG) coating can be used tomake a durable tool from a laser ablated polyimide roll. DLG isdescribed in U.S. patent application Ser. No. 11/185,078, filed Jul. 20,2005, which is incorporated herein by reference as if fully set forth. Afluoropolymer coating can also be used to improve the durability of anablated roll. Other materials for use as ablation substrates on a workpiece include polycarbonate, urethranes, and acrylates. The durabilityof the roll tool (work piece) can also be increased by coating it with athin layer of nickel, chrome, silver, or other material, which may alsoenhance its release characteristics.

The system can also be used to machine other materials such asnanocrystalline metals and fully dense ceramics, particularly metaloxides. However, these materials require about ten times the power forablation in comparison to the power required to ablate polymers.Ceramics can be ablated with the system; however, it can be difficult tomake a large roll of, or a roll covered with, a fully dense ceramicmaterial. Smaller rolls of ceramic material can thus be more desirablefor ablation.

The feature size that can be machined is determined by the wavelength oflaser light and the numerical aperture of the imaging optics. Thenumerical aperture is the sine of the vertex angle of the largest coneof meridional rays that can enter or leave an optical system or element,multiplied by the refractive index of the medium in which the vertex ofthe cone is located.

The machining of work piece 32 is accomplished by coordinated movementsof various components. In particular, the system, under control ofcomputer 12, can control movement of work piece in directions 33, 34,and 35 via drive units 36 and 38, while coordinating those movementswith control of laser 28 to provide a laser image onto the surface ofwork piece 32 for machining of it. The work piece surface can bestationary during the machining, or preferably it can be in motion withthe electronic and laser system delays accounted for by the computer toaccurately place the image in its desired location on the surface.

Work piece 32, after having been machined, can be used to make filmshaving the corresponding microstructures for use in a variety ofapplications. Examples of those films include optical films, frictioncontrol films, and micro-fasteners or other mechanical microstructuredcomponents. The films are typically made using a coating process inwhich a material in a viscous state is applied to the work piece,allowed to at least partially solidify, and then removed. The filmcomposed of the solidified material will have substantially the oppositestructures than those in the work piece. For example, an indentation inthe work piece results in a protrusion in the resulting film.

Laser Machining Process

As described above, the laser 28 and optics and projection mask 30typically remain stationary, while the work piece 32 rotates andtranslates axially in directions 33 and 34, respectively. An axis ofsmall range motion of the roll normal to the beam, direction 35, can behelpful to adjust the focus of the system. Alternatively, thisadjustment can be accomplished by moving the optics, the projectionmask, or both. For machining, the work piece is rotated and translatedaxially while computer 12 controls firing of laser 28 when work piece 32is in the correct position. Using this technique, a 12 inch (in)diameter roll that is 24 in long can be completely patterned in only fewhours for a shallow pattern.

One of the considerations for imaging on a roll based system forablation is the trade off between resolution and depth of focus. Thefollowing equations summarize that relationship:

${W = \frac{k_{1} \cdot \lambda}{NA}},$where W is the resolution, k1 is a constant, λ is the exposurewavelength, and NA is the numerical aperture, and

${{DOF} = \frac{k_{2} \cdot \lambda}{{NA}^{2}}},$where DOF is the depth of focus, k2 is a constant, and NA is thenumerical aperture.From these equations it can be determined that resolution can beimproved for a given wavelength of light only by increasing the NA ofthe system.

Since the system 10 images onto a curved surface, a geometric analysis,based upon the diagram shown in FIG. 3, illustrates the trade offs forsuch a system. As shown in FIG. 3, the value ΔX (56) represents thewidth of the image on an outer surface 53 of the work piece betweenpoints 50 and 51. The work piece has a radius R (52), and an angle θ(54) represents half the angular distance of imaged area 56. The valueΔZ (58) represents the vertical distance of the imaged area 56perpendicular to an outer surface of the work piece. Based upon thegeometry for imaging shown in FIG. 3, ΔZ=R−R*cos(θ) and ΔX=2*R*sin(θ).Table 1 summarizes exemplary values for this geometry. For a 12 indiameter roll, an image 5 mm wide would cover a vertical distance over20 microns (μm) making it unlikely that a suitable image could beproduced with certain DOF values. In addition, it is desirable toincrease the resolution of the system, which requires a reduction in theDOF.

TABLE 1 R (in) ΔX (mm) ΔZ (μm) 6.00 5.00 20.51 6.00 1.00 0.82 6.00 3.4910.00 10.00 5.00 12.30 10.00 1.00 0.49

Several approaches are possible to address the considerations relatingto imaging on a curved surface. A first approach to imaging on a curvedsurface involves an optical train that manipulates the excimer laserbeam into a long and narrow beam (e.g., 1 mm by 20 mm) rather than thesquare beam (5 mm by 5 mm) conventionally used. The projection maskswould then be long and narrow, but would not limit the range of patternsthat can be created with the system. As shown in FIG. 3 and summarizedin Table 1, a 1 mm wide beam (distance 56) that runs down the axis ofthe work piece would deviate less than 1 micron (distance 58) from aflat imaging plane for a 6 in diameter roll. In other cases, the opticalsystem or projection mask can be configured to provide for an image thatdeviates less than 20 microns or 10 microns in a distance perpendicularto the outer surface of the work piece. Therefore, when using very largecylinders, the size of the image field in the curved direction can belimited so that the laser image remains in substantial focus for imagingwith the focus error equal to ½ ΔZ (distance 58).

In a second approach, the final imaging optics can be designed toprocess the laser beam image in order to produce a cylindrical imageplane, in which case system 10 projects a flat image from the projectionmask onto a convex cylinder (the work piece). This approach curves theimage field in one direction, which involves positioning cylindricallenses in the imaging lens train. The amount of processing is typicallyrelated to an amount of curvature of the outer surface of the work piecein order to accurately project the flat (processed) image from theprojection mask onto the curved surface of the work piece. Such anapproach can be designed using ray tracing software, for example.Techniques for processing an image for projection onto a curved surfaceare described in U.S. Pat. Nos. 6,715,888 and 6,568,816, which areincorporated herein by reference as if fully set forth. This secondapproach of generating a curved image field is preferred for laserablation machining in that it results in the principal rays of the laserbeing perpendicular to the outer surface of the cylindrical work piecesuch that the rays “point” to the center axis of the work piece.

A third approach involves generating a curved image field using a curvedmask, such as an etched metal mask, with the amount of curvature of themask being related to the amount of curvature of the outer surface ofthe work piece.

While the present invention has been described in connection with anexemplary embodiment, it will be understood that many modifications willbe readily apparent to those skilled in the art, and this application isintended to cover any adaptations or variations thereof. For example,various types of lasers, imaging optics, masks, and materials to beablated may be used without departing from the scope of the invention.This invention should be limited only by the claims and equivalentsthereof.

1. A system for generating a laser machined tool, comprising: a laserproducing a laser beam image; a substantially cylindrical work piece,wherein the system is capable of rotating the work piece; and an opticalsystem for processing the laser beam image and for imaging the processedlaser beam image onto a convex outer surface of the work piece, whereinthe processing of the laser beam image is related to a curvature of theouter surface of the work piece, and the processing generates acylindrical image plane from the laser beam image with the principalrays of the laser beam image in the cylindrical image plane beingsubstantially perpendicular to the convex outer surface of the workpiece, and wherein the laser beam image is capable of ablating theconvex outer surface of the work piece in order to createmicrostructures within the surface to form a substantially cylindricaltool.
 2. The system of claim 1, further comprising a projection maskpositioned within the laser beam for shaping the laser beam into theimage.
 3. The system of claim 1, wherein the system is capable ofcoordinating the rotational movement of the work piece with activationof the laser in order to selectively ablate the outer surface of thework piece.
 4. The system of claim 3, wherein the system is capable ofmoving the work piece in a translational direction substantiallyperpendicular to the laser beam.
 5. The system of claim 4, wherein thesystem is capable of coordinating the rotational and translationmovements of the work piece with activation of the laser in order toselectively ablate the outer surface of the work piece.
 6. The system ofclaim 1, wherein the laser beam image is capable of ablating the surfacein a continuous spiral pattern on the tool.
 7. The system of claim 1,wherein the laser beam image is capable of ablating the surface in adiscontinuous spiral pattern on the tool.
 8. The system of claim 1,further comprising a material coated on the outer surface of the workpiece.
 9. The system of claim 8, wherein the material comprises apolymeric material.
 10. A system for generating a laser machined tool,comprising: a laser producing a laser beam image; a substantiallycylindrical work piece, wherein the system is capable of rotating thework piece; an optical system for processing the laser beam image andfor imaging the processed laser beam image onto a convex outer surfaceof the work piece; and a curved projection mask positioned within thelaser beam for shaping the laser beam into the image, wherein theprocessing of the laser beam image is related to a curvature of theouter surface of the work piece, and the amount of curvature of the maskis related to an amount of curvature of the convex outer surface of thework piece in order to perform the processing, and wherein the laserbeam image is capable of ablating the convex outer surface of the workpiece in order to create microstructures within the surface to form asubstantially cylindrical tool.
 11. The system of claim 10, wherein themask comprises an etched metal mask.
 12. The system of claim 10, whereinthe laser beam image has a deviation of less than 1 micron in adirection perpendicular to the convex outer surface of the work piece.